Against the backdrop of a rapid increase in demand for communications, studies are vigorously conducted for increasing communication network capacity. Amplitude-shift keying (ASK), in which a high-frequency electrical signal of one channel is allocated to light of one channel, was conventionally dominant as an optical modulation format. However, ASK can provide only a one-bit signal to a frequency band. Accordingly, in recent years, research and development of quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM) are actively conducted and these formats go into actual use.
In order to generate a QPSK or QAM signal, an IQ modulator that performs amplitude modulation individually for a real axis and an imaginary axis in a complex representation of light is generally used. The IQ modulator can modulate light of one channel by using high-frequency electrical signals of two channels for the real axis and the imaginary axis. Polarization multiplexing, in which different signals are provided to X polarization component of light and Y polarization component of light and transmitted, is also generally used. In the case of using both of IQ modulation and polarization multiplexing to improve frequency utilization efficiency for increasing communication capacity, light of one channel can be modulated by using high-frequency electrical signals of four channels.
As well as improving frequency utilization efficiency to increase communication capacity, activities are carried out for downsizing a transmission and reception device to increase transmission capacity per unit volume. If a device is downsized without changing transmission capacity per device, the number of devices mounted on a transmission apparatus can be increased, which leads to an increase in the total transmission capacity of the transmission apparatus.
However, if the number of channels for high-frequency electrical signals allocated to light of one channel is increased and an optical transmission and reception module is downsized, there is a problem that distances between high-frequency transmission lines through which electrical signals are transmitted become short and crosstalk between the high-frequency transmission lines becomes large (see, for example, Patent Literature 1).
FIG. 1A and FIG. 1B show the configuration of a conventional optical module 100. FIG. 1A is a top perspective view of the optical module 100 and FIG. 1B is a cross-sectional view along line IB-IB in FIG. 1A. The optical module 100 shown in FIG. 1A and FIG. 1B is disposed at the bottom of a housing 101 and covered with a lid 110. Here, FIG. 1A shows the optical module 100 from which the lid 110 is detached and FIG. 1B shows the optical module 100 to which the lid 110 is attached. The optical module 100 includes an optical processing circuit 103, an electro-optical transducer 104 connected to the optical processing circuit 103, and a lower substrate 109, which are disposed at the bottom of the housing 101. A lower ground 108 is formed on the lower substrate 109 and a high-frequency substrate 107 is formed on the lower ground 108. Four high-frequency transmission lines 105 connected to the electro-optical transducer 104 are formed on the high-frequency substrate 107. The four high-frequency transmission lines 105 constitute microstrip lines. The housing 101 is equipped with an optical port 102 and four electrical ports 106. The optical port 102 is connected to the optical processing circuit 103 and the four electrical ports 106 are connected to the high-frequency transmission lines 105, respectively.
The above configuration is generally applied to an optical module. The optical port 102, the optical processing circuit 103, the electro-optical transducer 104, the high-frequency transmission lines 105, and the electrical ports 106 constitute an optical signal transmission or optical signal reception module. Here, a signal flow is explained using an optical signal transmission module as an example. High-frequency electrical signals are input to the electrical ports 106 of the optical module 100. The high-frequency electrical signals are transmitted through the high-frequency transmission lines 105, converted into optical signals by the electro-optical transducer 104, multiplexed in the optical processing circuit 103, and then output as a wavelength-multiplexed optical signal from the optical port 102.
Since the high-frequency electrical signals are generally transmitted as an electromagnetic field expanded around the high-frequency transmission lines, crosstalk tends to be caused by interference between adjacent channels. Accordingly, as the housing 101 is downsized, the high-frequency transmission lines 105 are provided densely and the intervals become narrow, which results in a problem that crosstalk between the high-frequency transmission lines becomes large and affects the characteristics of transmitted signals.
The present invention has been accomplished in consideration of the conventional technique described above. The present invention aims to provide an optical module that suppresses crosstalk between high-frequency transmission lines.